Pre-clinical bone cancer pain models mimicking the human condition are required to respond to clinical realities. Breast or prostate cancer patients coping with bone metastases experience intractable pain, which affects their quality of life. Advanced monitoring is thus required to clarify bone cancer pain mechanisms and refine treatments. In our model of rat femoral mammary carcinoma MRMT-1 cell implantation, pain onset and tumor growth were monitored for 21 days. The surgical procedure performed without arthrotomy allowed recording of incidental pain in free-moving rats. Along with the gradual development of mechanical allodynia and hyperalgesia, behavioral signs of ambulatory pain were detected at day 14 by using a dynamic weight-bearing apparatus. Osteopenia was revealed from day 14 concomitantly with disorganization of the trabecular architecture (µCT). Bone metastases were visualized as early as day 8 by MRI (T1-Gd-DTPA) before pain detection. PET (Na18F) co-registration revealed intra-osseous activity, as determined by anatomical superimposition over MRI in accordance with osteoclastic hyperactivity (TRAP staining). Pain and bone destruction were aggravated with time. Bone remodeling was accompanied by c-Fos (spinal) and ATF3 (DRG) neuronal activation, sustained by astrocyte (GFAP) and microglia (Iba1) reactivity in lumbar spinal cord. Our animal model demonstrates the importance of simultaneously recording pain and tumor progression and will allow us to better characterize therapeutic strategies in the future.

Over the past few years, significant progress has been made in cancer therapy. Indeed, the lifespan of cancer patients has significantly increased. Although patients live longer, cancer-related pain remains a daily problem affecting their quality of life, especially when metastases reach the bone. In patients coping with cancer-induced bone pain, morphine and NSAIDs, often used in combination with other medications, are the most commonly used drugs to alleviate pain. However, these drugs have dose-limiting side effects. Morphine and other routinely used opioids are mu opioid receptor (MOPR) agonists. The MOPR is responsible for most opioid-related adverse effects. In the present study, we revealed potent analgesic effects of an intrathecally-administered selective delta opioid receptor (DOPR) agonist, deltorphin II, in a recently developed rat bone cancer model. Indeed, we found that deltorphin II dose-dependently reversed mechanical allodynia 14 days post-surgery in this cancer pain model, which is based on the implantation of mammary MRMT-1 cells in the femur. This effect was DOPR-mediated as it was completely blocked by naltrindole, a selective DOPR antagonist. Using the complete Freund's adjuvant model of inflammatory pain, we further demonstrated that deltorphin II was equipotent at alleviating inflammatory and cancer pain (i.e. similar ED50 values). Altogether, the present results show, for the first time, that activation of spinal DOPRs causes significant analgesia at doses sufficient to reduce inflammatory pain in a rat bone cancer pain model. Our results further suggest that DOPR represents a potential target for the development of novel analgesic therapies to be used in the treatment of cancer-related pain.

The analgesic potency of morphine-6-glucuronide (M6G) has been shown to be 50-fold higher than morphine after intracerebral injection. However, the brain penetration of M6G is significantly lower than morphine, thus limiting its usefulness in pain management. Here, we created new entities by the conjugation of the Angiopep-2 peptide (An2) that crosses the bloodbrain-barrier (BBB) by LRP1 receptor-mediated transcytosis, with either morphine or M6G. We demonstrated improvement of BBB permeability of these new entities compared with that of unconjugated M6G and morphine. Intravenous or subcutaneous administration of the An2-M6G conjugate exerted greater and more sustained analgesic activity than equivalent doses of either morphine or M6G. Likewise, subcutaneous An2-morphine induced a delayed but prolonged antinociceptive effect. The effects of these conjugates on the gastrointestinal tract motility were also evaluated. An2-morphine significantly reduced the intestinal transit time while An2-M6G exhibited a reduced constipation profile, as compared to an equimolar dose of morphine. In summary, we have developed new brain-penetrant opioid conjugates exhibiting improved analgesia to side-effect ratios. These results thus support the use of An2 carrier peptides as an innovative BBB targeting technology to deliver effective drugs such as M6G for the pain management.